Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Consequently, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx). Oxidation catalysts, such as diesel oxidation catalysts (DOC) have been implemented in exhaust gas aftertreatment systems to oxidize at least some particulate matter in the exhaust stream, reduce unburned hydrocarbons and CO in the exhaust to less environmentally harmful compounds, and oxidize nitric oxide (NO) to form nitrogen dioxide (NO2), which is used in the NOx conversion on an selective catalytic reduction (SCR) catalyst. To remove the particulate matter, a particulate matter (PM) filter is typically installed downstream from the oxidation catalyst or in conjunction with the oxidation catalyst. However, some exhaust aftertreatment systems do not have a PM filter. With regard to reducing NOx emissions, NOx reduction catalysts, including SCR systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. Further, some systems include an ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst to convert at least some ammonia slipping from the SCR catalyst to N2 and other less harmful compounds.
Exhaust aftertreatment system components can be susceptible to failure and degradation. Because the failure or degradation of components may have adverse consequences on the performance and emission-reduction capability of the exhaust aftertreatment system, the detection and, if possible, correction of failed or degraded components is desirable. In fact, some regulations require on-board diagnostic (OBD) monitoring or testing of many of the various components and performance of an exhaust aftertreatment system. When equipped on vehicles, most monitoring and testing of aftertreatment system components and performance are performed during on-road operation of the vehicle (e.g., while the vehicle is being driven on the road). Although such monitoring and testing while the vehicle is in use may be convenient, the efficacy of the monitoring and testing diagnostic procedures, as well as any recovery procedures, are limited because the engine cannot be operated outside of a given on-road calibrated operating range. Additionally, because on-road operating demands typically have priority over diagnostic and performance recovery procedures, the order, timing, and control of such procedures may be less than ideal.